This invention pertains to modified zinc-rich coatings and more particularly to those containing thermoplastic polyhydroxyether and zinc phosphosilicate.
Zinc-rich coatings for metallic substrates have increased in importance as replacements for paints containing toxic pigments based on lead or chromium compounds, such as red lead and zinc chromate pigmented paints. The performance of zinc-rich coatings applied directly on cold-rolled steel, however, is not completely satisfactory for many applications. It is used in combination with a chromium containing wash primer in the automotive industry for one-sided coated steel which is formed into parts and assembled by electrical spot welding. The salt spray resistance of zinc-rich coatings on cold-rolled steel also leaves much room for improvement.
It is therefore an object of this invention to provide zinc-rich coatings having electrical properties suitable for spot welding such as electrical conductance, and the like. In this regard aluminum is unsatisfactory despite its good electrical conductivity in bulk because its surface resistance is high due to oxidation under ambient conditions producing a tenacious oxide coating. As a result, aluminum powder has poor electrical conductance and high volume resistivity when used as a pigment in metal coatings.
It is a further object of this invention to provide metal coatings having good corrosion resistance, particularly on cold-rolled steel.